This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
A large volume of methane is currently contained in permafrost regions in the form of methane hydrates. In many cases, it may be desirable to recover the methane from the methane hydrates. Several techniques for recovering methane from methane hydrates have been explored. According to one technique, methane is recovered from methane hydrates via thermal stimulation. This may be accomplished by injecting high-temperature water into the hydrate layer through a pipeline. Another technique involves dissociating the methane from the methane hydrates via depressurization using a vacuum device. In addition, another technique involves dissociating the methane from the methane hydrates using inhibitors that cause the methane hydrates to become unstable.
U.S. Pat. No. 7,988,750 to Lee et al. describes a method for recovering methane gas from methane hydrates by adding a gas mixture containing nitrogen and carbon dioxide gases to the methane hydrates. Specifically, the methane within the methane hydrates is reacted with the gas mixture, and the gas mixture replaces the methane within the methane hydrates, thus releasing the methane. Furthermore, the gas mixture containing the nitrogen and carbon dioxide may be a flue gas obtained from a factory. However, the flue gas may include gas components other than nitrogen and carbon dioxide, such as water, sulfur, and a variety of other gas components. Thus, a cleanup apparatus may be used to protect the environment by removing the extra gas components from the flue gas prior to injection of the flue gas into the methane hydrates. In some cases, removing the gas components from the flue gas results in a significant increase in operating cost, thus rendering recovery of the methane from the methane hydrates less profitable.
A conventional gas turbine engine often has a turbine compressor that is mechanically linked to a turbine expander through a shaft. The turbine compressor can be used to compress a flow of air ingested by the turbine compressor. The compressed air is then passed to a combustor. In the combustor, fuel is injected and ignited to create a continuous flame. The high pressure exhaust gases from the flame are flowed into the turbine expander, which generates mechanical energy from the exhaust gas as it expands. The mechanical energy, transferred through the shaft to the turbine compressor, is used to power the compression of the air. Additional mechanical energy is produced, over the amount used to compress the ingested air, and harvested for other purposes, for example, to generate electricity. The flame temperature can exceed the metallurgical limits of the combustor can, so an excess amount of air is often used to provide cooling. However, this arrangement may create a higher amount of pollutants, such as nitrogen oxides (NOxs).
Capturing carbon dioxide from the exhaust gas for other uses may be problematic for a number of reasons. For example, there is a low concentration of carbon dioxide in the exhaust gas of a conventional gas turbine, and a very large volume of gas has to be treated. The exhaust gas may also be at a relatively low pressure, e.g., around 1050 kilopascals (kPa), and a relatively high temperature, e.g., from around 425 degrees Celsius (° C.) to around 700° C. Further, the exhaust gas may contain a large amount of oxygen that may interfere with CO2 extraction or use. Finally, the exhaust gas may be saturated with water from cooling, which can increase a reboiler duty in the CO2 extraction system.
The combustion of fuel within a combustor, e.g., integrated with a gas turbine, can be controlled by monitoring the temperature of the exhaust gas leaving the expander, because temperatures are generally too high in the combustor for existing instrumentation. At full load, typical gas turbines adjust the amount of fuel introduced to a number of combustors in order to reach a desired combustion gas or exhaust gas temperature.
However, controlling the amount of oxidant introduced to the combustor can also be desirable when an objective is to capture carbon dioxide (CO2) from the exhaust gas. Current carbon dioxide capture technology is expensive for several reasons. For example, the low pressure and low concentration of carbon dioxide in an exhaust gas. The carbon dioxide concentration, however, can be significantly increased from about 4% to greater than 10% by operating the combustion process under substantially stoichiometric conditions. Further, a portion of the exhaust gas may be recycled to the combustor as a diluent for cooling the products of combustion instead of air.
The enhanced exhaust gases may be captured for use by other systems, for example, directly from the exhaust of the gas turbine. However, if a gas turbine is being supplied an oxidant from a separate source, it may be more effective to compress the exhaust gas in the turbine compressor of the gas turbine, and recycle the compressed gas to the combustors as a coolant, then capture a high pressure bleed flow during the control of the recycle flow. Numerous studies have examined the concept of recycling a portion of the exhaust gases to the combustor.
For example, U.S. Pat. No. 4,271,664 to Earnest discloses a turbine engine with exhaust gas recirculation. The engine has a main power turbine operating on an open-loop Brayton cycle. The air supply to the main power turbine is furnished by a compressor independently driven by the turbine of a closed-loop Rankine cycle which derives heat energy from the exhaust gas of the Brayton turbine. A portion of the exhaust gas is recirculated into the compressor inlet during part-load operation. However, no additional uses are disclosed for the recycled exhaust gas.